Recently, considerable research and development work has been positively undertaken in progress on a solid electrolyte because the solid electrolyte has been considered to be of no fear in leakage of liquid and have specified ions that contribute to electric conductivity to be extremely effective as electronic material of a variety of devices such as cells and gas sensors.
Particularly, development work on a ceramic solid electrolyte fuel cell (SOFC) continues in progress, and a zirconia-based ceramic fuel cell with a capacity of several kW has achieved a record of operating performance for several thousand hours. Since the SOFC is operated at a high temperature (of >1000° C.) and hydrocarbon fuel can be reformed inside the fuel cell, a high combustion efficiency (of >60%) can be obtained.
In general, the SOFC is comprised of a solid electrolyte, an anode and a cathode and, in addition, an intermediate layer if desired. Such composition materials are required to be stable in oxidization/reduction atmosphere and to have suitable ionic conductivities. Such composition materials are also required to have their thermal expansion coefficients close to each other, and the anode and cathode are required to include porous bodies through which gas is permeable. Further, the composition materials of the cells are desired to be high in strength and toughness and low in costs and, further in view of stability in operation of the SOFC, to include material systems that are concurrently sintered to satisfy basic requirements for electric conductivity.
Now, material for the solid electrolyte includes stabilized ZrO2 that forms a mainstream in use, and as stabilizer, it has been usual practice to use oxides of alkaline earth metals, such as CaO, MgO, and rare earth oxides such as Sc2O3, Y2O3.
Here, ZrO2 doped with alkaline earth metal of CaO exhibits an ionic conducting characteristic value of 0.01 (S/cm) at 800° C. Further, it has been reported that the ionic conductivity of ZrO2 doped with one of rare earth oxides, such as Y2O3, Yb2O3 and Gd2O3, lies in a range of from 1×10−1 to 1×10−2 (S/cm) at 800° C. and decreases to a value less than 2×10−2 (S/cm) when temperature drops below 650° C. (see H. TANNENBERGER et al., Proc. Int' I Etude Piles Combust, 19-26 (1965)). Research and development work for stabilized zirconia doped with one of such rare earth oxides has been mainly started by 1970.
Additionally, as systems which are added with more than two kinds of rare earth oxides, a combination of two kinds of materials, selected from three moles of Y2O3—Yb2O3—La2O3, and three moles of Gd2O3—Yb2O3 have been reported (see Japanese Patent Application Laid-Open Publication No. 06-116026). However, a characteristic value of such systems is not so high as 0.005 (S/cm) at 1000° C.
Recently, zirconia stabilized with scandium oxide has been disclosed as a solid electrolyte for a fuel cell operable at a temperature higher than 700° C. (see Japanese Patent Application Laid-Open Publication No. 06-150943 and Japanese Patent Application Laid-Open Publication No. 10-097860). However, since the strength of zirconia material sharply drops at a temperature in the vicinity of 600° C., development work has been undertaken mainly for technique of adding element, such as Al2O3, to the base material.
Now, in a solid electrolyte cell, power output of a unit cell is typically limited to a value of approximately 1.1 volts, and in order to obtain high power output, the solid electrolyte cell is required to take a laminated structure or a parallel structure. However, a ceramic cell with such a structure becomes large in size, making it extremely hard to select a particular system of ceramic materials while causing a difficulty in establishing technology to manufacture a large-sized fuel cell. To address such an issue, a container of a combustor body of such a large-sized ceramic fuel cell is required to effectively utilize a metallic component part, such as stainless steel of ferrite system. In order to effectively utilize such metal, the fuel cell needs to use solid electrolyte materials which are active throughout a wide temperature range, especially even at low temperatures (in a range of from 600° C. to 800° C.) so as to have an ionic conductivity equivalent to that resulting at the high temperature (of >1000° C.).
Further, the solid electrolyte has crystal that tends to break at temperatures around 650° C. Accordingly, when applied to the oxygen sensor, no need arises so far to take influence of a grain boundary phase occurring at the temperature, range described above and influence as a result of aging seriously. In contrast, when applied to the fuel cell, it has been strongly required to establish technology for stabilizing a crystal phase of the solid electrolyte at such a temperature range or at a further increased temperature range and to prevent the solid electrolyte from deterioration in strength at such high temperatures under the oxidation/reduction atmosphere. In this regard, a method of adding Al2O3 for stabilizing a crystal structure is disclosed (see Japanese Patent Application Laid-Open Publication No. 05-225820).
Furthermore, with respect to a method of manufacturing ceramic material, it has been proposed to provide a sintering method using a spark plasma-sintering machine (see Japanese Patent Publication No. 3007929 and its corresponding Japanese Patent Application Laid-Open Publication No. 10-251070).